1. Field of the Invention
The present invention relates to an air/fuel ratio detector for use in the measurement or control of the concentration of oxygen in exhaust gas from a combustion device such as an internal combustion engine or gas burner.
2. Description of the Prior Art
One type of oxygen sensor is capable of detecting combustion occurring at nearly a theoretically optimal or stoichiometric air/fuel ratio by sensing a change in electromotive force that is produced by the difference between the partial oxygen pressure of the exhaust gas and that of the atmospheric air. This type of oxygen sensor comprises an ion-conductive solid electrolyte (e.g., stabilized zirconia) coated with porous electrode layers (e.g., porous layers of Pt). This device is presently used in several applications, for example, in an automobile for the purpose of running its internal combustion engine at the theoretical air/fuel ratio.
The conventional oxygen sensor produces a great change in its output if the operating air/fuel ratio (A/F ratio, which is the weight ratio of air to fuel) is near the stoichiometric value of 14.7 but, otherwise, the resulting change in output is negligibly small. Therefore, the output from this sensor cannot be effectively used if the engine is operating at A/F ratios other than near the stoichiometric value.
Japanese Patent Application (OPI) No. 153155/1983 (the symbol OPI signifying an unexamined published Japanese patent application) shows an oxygen concentration detector comprising a pair of oxygen ion conductive and solid electrolyte plates each having an electrode layer on both sides in a selected area close to one end of the plates. The two plates are fixed parallel to each other and spaced to leave a gap in an area corresponding to that selected area having the electrode layers. One electrolyte plate with electrode layers is used as an oxygen pump element. The other plate, also having electrode layers, is used as an electrochemical cell element that operates by the difference in oxygen concentration between the ambient atmosphere and the gap between the two plates. This type of detector features quick response. However, according to experiments conducted by the present inventors, if this device is used in a fuel-rich region having a A/F ratio lower than the stoichiometric 14.7 value, the direction of change in the resulting output relative to the signal for the stoichiometric mixture is the same as that obtained in a fuel-lean region as shown in FIG. 8. Because of the existence of two A/F ratios for a single output, the sensor can be used only when it is definitely known whether the device to be controlled is operating in the fuel-rich or the fuel-lean region. Furthermore, the present inventors found it difficult to detect an A/F ratio at or near the stoichiometric value of 14.7, or accurately and responsively enable a feedback control over the A/F ratio by using this device.